1. Field of the Invention
The present invention relates to a composite sheet for electric insulation using as matrix material an epoxy type resin composite which hardens to form a sheet excelled in flexibility, cracking resistance, antihygroscopicity and electric insulation property, and this composite sheet for electric insulation is useful as an insulation material for use in dry transformers, motors, generators and the like.
2. Description of the Related Art
Epoxy type resins with their high mechanical strength, excellent heat resistance and electric insulation property are widely used as molding materials for electric and electronic parts/components and the like. Ordinary epoxy resin, however, generally lack in flexibility and less suited as materials for films and sheets.
Meanwhile, as a sheet material for electric insulation there is known a product, "Nomex" by trade name, marketed by DuPont. This sheet material, being short-length fibers based on meta-aramid polymer of the following structural formula ##STR2## filtered and formed paper-like having an excellent heat resistance, electric insulation and mechanical strength and is being widely used as heat resistant insulation sheet.
This insulation sheet has, besides its extremely high cost, drawbacks of being hygroscopic and its electric insulation property being lowered after a relatively short period of use. Worse, made by filtering short-length fibers, the sheet has therein countless interfiber voids, this interfering with the material's electric insulation property, and has something to be desired about its dielectric breakdown strength.
The present invention has been made in view of such situation and is aimed at provision of a composite sheet excelled in anti-hygroscopicity, heat resistance, flexibility and electric insulation property having as its matrix component an epoxy-type resin composite which affords a polymer excelled in heat resistance, insulation property, anti-hygroscopicity et cetera and also in flexibility.